This invention relates to radiometric temperature measurement systems (also known as xe2x80x9cpyrometersxe2x80x9d) and more particularly to a measurement method employing a pyrometer system having improved low temperature measurement sensitivity for determining a surface temperature of a semiconductor wafer or an object without contacting its surface.
Pyrometer-based temperature measurement systems have a long development history. For example, even before 1930, U.S. Pat. Nos. 1,318,516; 1,475,365; and 1,639,534 all described early pyrometers. In 1933, U.S. Pat. No. 1,894,109 to Marcellus described a pyrometer employing an optical xe2x80x9clightpipe.xe2x80x9d In 1955, U.S. Pat. No. 2,709,367 to Bohnet described a pyrometer in which sapphire and curved sapphire lightpipes are used in collection optics. In 1971, U.S. Pat. No. 3,626,758 to Stewart described using quartz and sapphire lightpipes with a blackbody sensor tip. Then in 1978, U.S. Pat. No. 4,075,493 to Wickersheim described a modern flexible fiber optic thermometer.
In the 1980s, U.S. Pat. No. 4,348,110 to Ito described electronic improvements to pyrometers, such as an integrating photo-detector output circuit. Then U.S. Pat. Nos. 4,576,486, 4,750,139, and 4,845,647, all to Dils, described further improvements to electronics, fiber-optics, sapphire rods, and blackbody emission temperature measurements.
In the 1990s, many patents issued that describe the use of pyrometers in semiconductor processing. For example, in 1990, U.S. Pat. No. 4,956,538 to Moslehi described using fiber optic lightpipes for wafer temperature measurements in rapid thermal processing (xe2x80x9cRTPxe2x80x9d) applications. In 1992, U.S. Pat. No. 5,154,512 to Schietinger described using a fiber optic thermometer with wavelength selective mirrors and modulated light to measure semiconductor wafer temperatures. In 1998, U.S. Pat. No. 5,717,608 to Jenson described using an integrating amplifier chip and fiber-optics to measure semiconductor wafer temperatures, and U.S. Pat. No. 5,815,410 to Heinke described an infrared (xe2x80x9cIRxe2x80x9d) sensing thermometer using an integrating amplifier. Then in 1999, U.S. Pat. No. 5,897,610 to Jensen described the benefits of cooling pyrometers, and U.S. Pat. No. 6,007,241 to Yam described yet another fiber optic pyrometer for measuring semiconductor wafer temperatures.
As one can see from these prior patents, pyrometer systems are commonly used for measuring the temperature of semiconductor silicon wafers housed within a process chamber while forming integrated circuits (xe2x80x9cICsxe2x80x9d) on the wafer. Virtually every process step in silicon wafer fabrication depends on the measurement and control of wafer temperature. As wafer sizes increase and the critical dimension of very large scale ICs scales deeper into the sub-micron range, the requirements for wafer-to-wafer temperature repeatability during processing become ever more demanding.
Processes such as physical vapor deposition (xe2x80x9cPVDxe2x80x9d), high-density plasma chemical vapor deposition (xe2x80x9cHDP-CVDxe2x80x9d), epitaxy, and RTP can be improved if the wafer temperature is accurately measured and controlled during processing. In RTP there is a special importance to temperature monitoring because of the high temperatures and the importance of tightly controlling the thermal budget, as is also the case for Chemical Mechanical Polishing (xe2x80x9cCMPxe2x80x9d) and Etch processes.
As wafer sizes increase, the cost of each wafer increases geometrically, and the importance of high quality in-process temperature monitoring increases accordingly. Inadequate wafer temperature control during processing reduces fabrication yields and directly translates to lost revenues.
In addition to conventional pyrometry, the most common in-situ temperature sensing techniques employed by semiconductor processing wafer fabs and foundries also includes thermocouples and advanced pyrometry.
Thermocouples are easy to use, but their reliability and accuracy are highly questionable. Thermocouples are only accurate when the wafer is in thermal equilibrium with its surroundings and the thermocouple is contacting or embedded in that environment. Otherwise, the thermocouple reading might be far from the correct wafer temperature. For example, in PVD applications, while the thermocouple embedded in the heated chuck provides a temperature measurement that resembles that of the wafer, there are large offsets between the wafer and the thermocouple. These offsets are a function of gas pressure and heat transfer.
In conventional optical pyrometry, a pyrometer deduces the wafer temperature from the intensity of radiation emitted by the wafer. The pyrometer typically collects the radiation from the wafer through an interface employing a lens or a quartz or sapphire rod. Such interfaces have been used with PVD, HDP-CVD, RTP, Etch, and rapid thermal chemical vapor deposition (xe2x80x9cRTCVDxe2x80x9d). While conventional optical pyrometers are often superior to the use of thermocouples, there are measurement inaccuracy problems caused by the processing environment, such as background light, wafer transmission, emissivity, and signal-to-noise ratio problems that cannot be ignored.
Advanced pyrometry offers some satisfactory temperature monitoring solutions for semiconductor wafer production applications. xe2x80x9cOptical Pyrometry Begins to Fulfill its Promise,xe2x80x9d by Braun, Semiconductor International, March 1998, describes advanced pyrometry methods that overcome some limitations of conventional pyrometry. As such, optical pyrometers and fiber optic thermometers employing the Planck Equation are now commonly used for in-situ semiconductor wafer measurement. However, numerous problems and limitations are still encountered when measuring wafer temperature using xe2x80x9cPlanckxe2x80x9d radiation (light) emitted by the wafer. There are numerous problems when measuring wafers at temperatures below about 400xc2x0 C.: 1) minimal signal levels generated by the photo detector (as small as 1E-16 amps) because the very small amount of radiation emitted by the wafer; 2) the wafer is semi-transparent at low temperatures and long wavelengths (greater than 900 nm); and 3) the background light is often larger than the emitted wafer signal and causes large errors when it enters the collection optics. Moreover, electromagnetic radiation transmission losses and the emissivity of the object being measured is often unknown and changing, which increases the difficulty of achieving accurate temperature measurements.
What is still needed, therefore, is an advanced pyrometer system and measurement method that provides accurate and repeatable temperature measurements of an object, such as a semiconductor wafer, down to about room temperature without contacting the object being measured.
An object of this invention is, therefore, to provide an apparatus and a method for performing non-contacting temperature measurements of target media.
Another object of this invention is to provide an advanced pyrometer system capable of measuring semiconductor wafer temperatures down to or below about room temperature.
A further object of this invention is to provide an advanced pyrometer system and method for making high accuracy semiconductor wafer temperature measurements in a plasma processing environment.
An advanced pyrometer system of this invention has reduced optical losses, better background radiation blocking, improved signal-to-noise ratio, and improved signal processing to achieve improved accuracy and temperature measurement capabilities ranging from about 10xc2x0 C. to about 4,000xc2x0 C.
The system includes collection optics that acquire emitted radiation from a hot specimen and directly couples it to an optional filter and a photo detector. The collection optics may include lens systems, optic lightpipes, and flexible fiber optics. The preferred collection optic is a yttrium-aluminum-garnet (xe2x80x9cYAGxe2x80x9d) light guide rod. The filter or filters employed are wavelength-selective to determine which radiation wavelengths are measured, and optionally includes a hot/cold mirror surface for reflecting undesired radiation wavelengths back to the specimen. The photo detectors are formed from silicon, InGaAs or, preferably, doped AlGaAs having narrow bandpass detection characteristics centered near 900 nm. The doped AlGaAs detector allows eliminating the optical filter, in some applications, if additional detection sensitivity is required. Also, light at wide angles has less effect on the wavelength sensitivity of the AlGaAs detector.
The system further includes an amplifier that acquires and conditions signals as small as 10-16 amps for detection and measurement. A signal processor converts the amplified signal into a temperature reading. This processing is a combination of electrical signal conditioning, analog-to-digital conversion, correction factors, and software algorithms, including the Planck equation.
In a preferred embodiment, the temperature of a semiconductor wafer is measured while undergoing processing in a plasma environment. At least two pyrometers receive radiation from, respectively, the semiconductor wafer and background light from a plasma in a plasma etch, strip, or deposition process chamber. The first pyrometer receives radiation from either the front or rear surface of the wafer, and the second pyrometer receives radiation from the plasma. In this embodiment, both pyrometers may be sensitive to the same radiation wavelength.
A central processor unit or controller receives signals from the first and second pyrometers and calculates a corrected wafer emission, which is employed in the Planck Equation to calculate the wafer temperature.
In an alternate embodiment of this invention, both pyrometers are positioned beneath the wafer. The first pyrometer is sensitive at a first wavelength where the wafer is substantially opaque to plasma radiation, and the second pyrometer is sensitive at a wavelength where the wafer is substantially transparent to plasma radiation.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.